Furnace brazing generally refers to processes for brazing together metal parts which have a melting point appreciably above copper. The metal parts are pre-positioned into an assembly which is passed through the brazing furnace wherein a preformed copper brazing part is melted and drawn into a close fitting joint by capillary action to make a strong brazed joint. More recently, brazing pastes have been developed for brazing metal parts together. Brazing pastes are copper powders dispersed in a vehicle to provide a paste consistency which can be easily applied to any shaped metallic joint which eliminates the need for a variety of preformed brazing parts. Examples of typical brazing pastes may be found in a series of Klinker patents such as, for example, U.S. Pat. No. 2,566,339; U.S. Pat. No. 2,594,313; and U.S. Pat. No. 2,606,132; and the disclosures of said patents are incorporated herein by reference.
Prior to this invention, conventional copper brazing pastes were often based on copper oxide powders which reduce to copper metal during the brazing process. U.S. Pat. No. 2,606,132, for example, recommends copper oxide in combination with minor amounts of finely divided iron. Copper oxides have been preferred in the past since the same usually contain less impurities and can be ground to a fine powder, that is, less than four microns if so desired. Copper oxides, however, contain about 11% oxygen which is a limiting factor in most furnace brazing processes due to conventional use of a poor reducing exothermic gas. Although copper powders made by electrolysis from solutions or gaseous reduction of copper oxide have been suggested for copper brazing pastes, such copper powders were less desirable than copper oxide primarily since such copper powders generally have more non-reducible oxidic impurities than copper oxide. The impurities in electrolytic copper like copper oxide do not completely reduce to metallic state in the exothermic reducing gas used in furnace brazing. Non-reducible oxidic impurities include, for example, SiO.sub.2, CaO, MgO, ZnO, Al.sub.2 O.sub.3, and the like which tend to become complexed with copper oxide during gaseous reduction and detrimentally inhibits copper coalescence. Impure copper powders and copper oxide powders leave undesirable amounts of black residue which must be removed from the brazed part, especially when the part is subsequently plated. Fluxes such as H.sub.3 BO.sub.3, KNO.sub.3, NaF, and KbF.sub.4 have been suggested to overcome the undesirable adverse effects of oxide impurities. However, the use of fluxes is not preferred since fluxes tend to damage furnace lining, conveyors, trays, and silicon carbide heating elements.
It now has been found that atomized copper powders are extraordinarily pure copper powders substantially free from oxide impurities and are particularly suitable for use in copper brazing pastes.
Accordingly, a primary object of this invention is to provide copper brazing paste based on atomized copper powder that is exceptionally pure and containing not substantially more than about 0.1% non-reducible impurities.
A further object is to provide a copper brazing paste based on atomized copper combined with minor amounts of very finely divided metal powders and/or reducible metal oxides and dispersed in a suitable fugitive vehicle.
A further object of this invention is to provide a brazing paste containing a controlled size distribution of atomized metallic copper being substantially spherical and particularly suitable to provide a residue-free brazing paste adapted to achieve clean brazed areas on assembled parts.
A further object of this invention is to provide brazing paste wherein impurities, if any, are situated substantially on the surface of the atomized copper particles rather than distributed uniformly through the copper particle.
These and other advantages of this invention will become more apparent by referring to the Detailed Description of the Invention.